The present invention relates to an arrangement according to claim 1 in a boom system used in a forest machine, in particular. This type of boom system comprises at least two interlinked boom parts and at least one power transmission means connected to the boom parts for moving the boom parts in relation to each other and the environment.
In a forest machine, a boom system is used for controlling various tools, such as a cutting head of a harvester or a timber grabbing bucket of a forwarder. The boom system then comprises two or more interlinked boom parts, the movement of which is controlled by actuators that are connected to lugs or corresponding fastening means attached to the boom parts for this purpose. These fastening means are arranged to transmit the desired force effect or support reaction to the structure of the boom part. In the case of a forest machine, these actuators forming the power transmission means are typically pressure medium-operated actuators producing a linear push or pull movement. Preferably, the actuators comprise one or more hydraulic cylinders or structures, such as pull or push rods, which transmit the movements of the same. The actuators can also be formed of a combination of said hydraulic cylinders and pull or push rods.
The boom parts are conventionally made of box-type structures assembled by welding from steel plates or pipe, to which the necessary fastening elements are attached mainly by welding. However, each weld joint in a boom part or made to it weakens its fatigue resistance and thus shortens the service life and reliable operation of the boom system. An attempt to solve this problem is to select stronger materials with a higher material strength for use in manufacturing the boom part. This way, the boom part is better able to endure the loads directed to it. The reverse side of such solutions is, however, that they increase the weight of the boom system, which in turn weakens the performance of the boom system.
Making welded structures is also time- and labour-consuming due to the difficult features related to the manufacturing method. For instance, deformation forces are during welding directed to the lugs required to fasten the actuator and, as a result of these, the lugs need to be machined after welding to achieve a dimensionally accurate end-result. In some cases, this problem has been solved by making the lugs from cast pieces fastened to the boom part by a bolted joint. However, problems related to these include the loosening of the bolts and the need for maintenance of the structure during use, for instance.